The invention relates to a display device, and in particular to pixels within display devices.
In general, thin film transistors (TFTs) applied in panel display devices can be divided into two categories, amorphous silicon (a-Si) TFT and low temperature poly-silicon (LTPS) TFT. Electron mobility of the LTSP TFT is 100 times higher than that of the a-Si TFT, capable of outputting enough current to light an organic light-emitting diode (OLED). When the a-Si TFT generating insufficient current is applied in an active OLED, a large voltage must be supplied to the a-Si TFT for generating larger current, resulting in undesirable rapid quality degradation thereof. Thus, the LTPS TFT is often applied in active OLED display devices.
A conventional active OLED display device comprises a plurality of pixels, each pixel PIX shown in FIG. 1 is composed of at least two LTPS TFTs. A transistor T1 is serially coupled to a light-emitting device (LED) D between voltage sources Vdd and Vss. A gate of a transistor T2 receives a scan signal Vscan through a signal line while a drain thereof receives a data signal Vdata through a data line. When the transistor T2 is turned on by scan data Vscan, data signal Vdata corresponding to the pixel PIX is transmitted to a gate of the transistor T1. When the pixel PIX is designed to emit light, the transistor T1 is turned on by the potential of the data signal Vdata and generates a driving current I, so that the LED D emits light. At the same time, a capacitor C stores a voltage Vgs related to the driving current I. When the transistor T2 is turned off by the scan data Vscan, the transistor T1 continues generating the driving current I due to the voltage Vgs of the capacitor C, so that the LED D continues to emit light.
In the LTPS TFT fabrication process, a crystal step is performed with a laser. Since the width of the laser beam is limited, the laser is not able to irradiate all TFTs at a time. Thus, by repeating the crystal step, each TFT can be irradiated.
The intensity of each leaser beam, however, differs, and the TFTs irradiated by different leaser beams have different threshold voltages. If the threshold voltages of the transistors T1 within all pixel PIX drifts, the driving currents I generated by the transistors T1 are differ, resulting in non-uniform brightness. Thus, it is difficult to design a display panel capable of uniformly emitting light with pixel circuit in FIG. 1.
Additionally, each pixel is coupled to the voltage source Vdd through a power line. The longer the power line, the larger the parasitical resistance thereof. Thus, pixels near the voltage source Vdd are brighter, while pixels farther from the voltage source Vdd are darker.